The present specification relates to method of making a semiconductor switch device and to a semiconductor switch device.
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) can be used as electric switches for high Radio Frequency (RF) signals. For these applications the device does not actually switch at high frequencies, rather it passes or blocks RF signals. Best performance is achieved with a device that combines low series resistance in on-state with low input capacitance in off-state. One of the most popular ways to evaluate the overall performance of an RF Switch MOSFET is to calculate a figure of merit (FOM), which is defined as the product of the on-state resistance (Ron) and the off-state capacitance (Coff).
Contributions to the off-state capacitance are the gate-to-source (Cgs), gate-to-drain (Cgd), gate-to-substrate (Cgb), drain-to-substrate (Cdb), and source-to-substrate (Csb) capacitance as shown in the following formulas:
            C      off        =                                        (                                          1                                                      C                    dd                                    -                                      C                    ds                                                              +                              1                                                      C                    ss                                    -                                      C                    ds                                                                        )                                -            1                          +                  C          ds                    =                        (                                                    C                dg                            *                              C                gs                                                                    C                dg                            +                              C                gs                                              )                +                  (                                                    C                db                            *                              C                sb                                                                    C                db                            +                              C                sb                                              )                +                  C          ds                                C      ss        =                  C        ds            +              C        gs            +              C        sb                        C      dd        =                  C        ds            +              C        gd            +              C        db            
Here, Cds is the channel capacitance, and Csb, Cdb are the source to bulk, drain to bulk capacitances, respectively. The overlap capacitance (Cgs=Cgd) and parasitic junction capacitances (Cdb=Csb) are of great importance as they constitute both significant fractions of the total off-state capacitance (Coff). Therefore, to achieve low Coff, it is desirable to reduce the overlap capacitance and/or the parasitic junction capacitances.